This invention relates generally to amplifiers and, more particularly, to a complementary metal oxide semiconductor (CMOS) operational amplifier characterized by high speed and high gain.
The advantages offered by CMOS technology over bipolar technology are well known (e.g. higher density, lower power, greater yield, etc.). These characteristics are extremely important in the design and fabrication of complex digital integrated circuits; for example, single chip microprocessors.
Whereas digital circuitry is generally characterized by its "on/off" or "1/0" nature, most measurements in the real world are inherently analog; e.g., temperature, pressure, speed, voltage, etc. Therefore, it is necessary that microprocessors and other digital circuitry communicate or interfacing with analog circuitry such as amplifiers, buffers, comparators, etc. in order to permit digital processing of the analog signals. The required interfacing may be accomplished by providing analog components which are external to the microprocessor chip. However, such arrangements generally require more current, a larger power supply and commonly present more opportunities for design and manufacturing errors. To avoid these disadvantages, complex analog circuits such as operational amplifiers are being manufactured integrally with the digital circuitry; e.g., on the microprocessor chip itself, and due to the complex nature of microprocessors, the inclusion of analog devices on the same chip requires that the same manufacturing process be employed. Thus, an operational amplifier included on a CMOS microprocessor chip must be fabricated in accordance with CMOS processing techniques, and the design of the operational amplifier must be tailored to such processing techniques.
CMOS operational amplifiers have been characterized as having significantly slower speeds than their bipolar counterparts and, in addition, have less gain. In fact, increasing gain in accordance with prior art techniques generally requires further reductions in speed; i.e. to increase gain the amplifier must be operated at lower current which means higher impedances and therefore longer time constants. In the past, performance speed has been increased at the expense of gain. The gain could in turn be increased by cascading amplifiers; however, such an arrangement is not suitable for closed loop applications due to excess phase shift resulting in an unstable circuit.